1. Field of the Invention
The present invention relates to a compact lighting system having a high illumination and a uniform luminous distribution characteristic.
2. Description of the Related Art
A light emitting diode (LED) has an advantage that it is compact and the power consumption is small and therefore, the lifetime is more than ten times that of a fluorescent light. Incorporation of a condenser lens into this LED allows approximately 90% of the emitted lights to be projected ahead without an additional, specific reflector. This type of lighting system can project light having extremely strong directivity and high luminance. There is developed a large-sized LED (power LED) a light emitting area of which is larger than conventional and which has extremely high luminance. There are studies on an application of such a power LED in various fields as a compact light source for illumination in low consumption power instead of a conventional incandescent lamp or a fluorescent light.
Techniques with respect to LEDs applicable to such a light source for illumination are proposed by Japanese Patent Application Laid-open No. 9-069561 (1997), Japanese Patent Application Laid-open No. 2002-049326, Japanese Patent No. 3118798 and so on. Japanese Patent Application Laid-open No. 9-069651 (1997) discloses a semiconductor light emitting module capable of increasing reliability and having no variation in characteristics by avoiding characteristic degradation and characteristic defect due to bonding of a lead thin line by using a selected light emitting diode device. Japanese Patent Application Laid-open No. 2002-049326 discloses a planar semiconductor light emitting device in which optical elements for collimation having an array corresponding to an array of LED chips are arranged as a micro lens array. By the planar semiconductor light emitting device, a large part of lights emitted by individual LED chips can be projected within an extremely narrow range. Japanese Patent No. 3118798 discloses a semiconductor light emitting module in which outer lenses are located as opposed respectively to light emitting diodes arranged by predetermined intervals, thereby making it possible to illuminate a larger area.
The power LED having a light emitting area larger than that of the conventional LED has a relatively large variation in light emitting luminance for each product. Therefore, in order to produce a product having a light emitting luminance within a predetermined tolerance, the manufacturing yield is only several dozens of percentages according to the current technology. Accordingly, in a case where this power LED is used as a light source for illumination requiring a uniform luminous distribution characteristic, it leads to an extremely high cost due to the low manufacturing yield as described above.
In the semiconductor light emitting module disclosed in Japanese Patent Application Laid-open No. 9-069651 (1997), selection of light emitting diode device is made for avoiding variations in characteristic. Therefore, as a result of basically eliminating use of a light emitting diode device exceeding a predetermined tolerance, the manufacturing yield of light emitting diodes deteriorates in the same way as in the case of the power LED, leading to an increase of a manufacturing cost of the semiconductor light emitting module.
The planar semiconductor light emitting device disclosed in Japanese Patent Application Laid-open No. 2002-049326 locates optical elements for collimation having an array corresponding to an array of the LED chips, as a micro lens array. Therefore, there occurs illumination unevenness in response to the variation in light emitting luminance of the individual LED chip. Yet since a LED chip array having an area in accordance with the illumination region is required, in a case of illuminating a large region in an uniform luminous distribution characteristic, a large amount of LED chips corresponding thereto are required to be used. Accordingly, this light emitting device is not nearly practical in terms of costs.
In the semiconductor light emitting module disclosed in Japanese Patent No. 3118798, the respective light emitting diodes and outer lenses are designed to separately illuminate only a part of the entire illumination region. This is apparent because the light emitting diodes and the outer lenses are arranged coaxially. As a result, in the same way as in the case of Japanese Patent Application Laid-open No. 2002-049326, the variation in the light emitting luminance of each light emitting diode leads directly to illumination unevenness in the illumination region, so that the uniform luminous distribution characteristic, i.e., the uniform illumination distribution can not be basically obtained. In addition, in a case of selecting the light emitting diodes for avoiding this problem, the manufacturing cost increases.